1. Field of the Invention
The present invention relates to an improvement of a photographing lens system.
2. Prior Arts
Hitherto, such a conventional focussing system of a camera has been known that the camera having a permanently mounted lens (not in an interchangeable manner) and distance is measured by means of optical means or ultrasonic means built in a camera body and the lens is driven by an electric motor or an electromagnetic means so as to automatically to focus the lens to the measured distance.
However, in such conventional automatic focussing system, the system was usually designed for the lens which was permanently mounted on the camera body, and therefore, a known utility of interchanging use of several lenses to a camera could not be utilized. Therefore, for single lens reflex type camera, another focussing system has been proposed. That is, distance measurement is made by means of TTL (through the lens) type measuring sensors disposed in the path of light after the lens, thereby enabling interchanging of the photographing lens.
FIG. 1 shows a schematic sectional view of such conventional system provided in the single lens reflex type camera. Light which passed the photographing lens 1 is reflected by a moving mirror 2 to the finder part 21, and a part of the light is reflected by a sub-mirror 3 to photoelectric sensors 5 and 5', through an electrically vibrating lattice 7 and a condenser lens 8. In this known system, a focussing of the photographic image is made when the phases of the output signals from the sensors 5 and 5' become the same, and a loss of the focussing is detected by that the phases become different from each other. The outputs of the sensors 5 and 5' are lead to a differential amplifier 6 and the latter's output energizes LED 10 which can be observed for instance in the field of finder. In such system, since the focussing information system operates with the light after passing the photographing lens, the focussing information system can be used for any of interchangeable lens. However, such conventional system has a defect that the sub-mirror 3 must have a complicated mechanism for quick removal from the light path at a photographing moment in a very minute space. Furthermore, in order to lead a part of light to the sensors 5 and 5', the main mirror 2 must have a half-mirror (i.e., semi-reflecting and semi-transparent part) at the center thereof, and therefore, the light given to the finder 21 is somewhat weakened at the central part, which is likely to result in a defect of difficulty of observation of the photographic object at the center part.
On the other hand, if a distance measurement of device using an ultrasonic wave were built in the camera body of a lens-interchangeable camera as a common use for interchanged lenses, the device must has a means for adapting to differences between focal lengths or picture angles of different lenses, and such means to compensate the differences would become very complicated.